The present invention relates generally to printers which utilize a print head carriage to print characters and, more particularly, to a method and apparatus for monitoring movement of the print head carriage to efficiently and accurately control such printers.
In the field of printing, the most common type printer has been the printer which impacts a record media which is stepped through the printer in a line-by-line motion as lines of print are formed on the media. The impact printing operation depends upon the coordinated movement of impact members, such as print hammers, wires or the like which are supported upon a print head, and a print head carriage which supports the print head and is moved back and forth across the record media. Movement of both the impact members and the print head carriage are controlled by electromechanical drive systems which enable their precise control.
To increase the printing speed and performance of printers, the size of the printing elements, the print head and the print head carriage have been continually reduced such that they can be moved and controlled more rapidly and accurately. In addition to the size of the printing elements and print head per se, the method and apparatus used to monitor the position of the print head can influence the operation of a printer. For example, the mass of the print head is typically increased by the apparatus used to monitor its position.
U.S. Pat. No. 4,898,487 discloses a print head carriage for a matrix printer wherein an optical sensor is mounted directly onto the print head carriage and moved along with the carriage. The optical sensor is used to monitor teeth of a comb-like timing strip which forms a portion of the printer frame and is positioned such that its teeth extend into the optical sensor. In this way, as the print head carriage is moved back and forth across the record media, a series of pulses is produced. The pulses cumulatively represent the position of the print head carriage and are monitored to control the printer. The ends of the timing strip are not toothed such that they define home positions on either side of the printer.
While the print head carriage monitoring arrangement of this patent provides reliable and accurate printer control, unfortunately, the optical sensor adds to the mass of the print head carriage and requires additional electrical connections to the print head carriage to conduct the signals generated by the optical sensor.
In the interest of further improving printing speed and performance, there is a need for an improved method and apparatus for monitoring the print head carriage of a printer. Preferably, the monitoring method and apparatus would provide accurate carriage position information without the addition of substantial mass to the print head carriage and without requiring any electrical connections to the print head carriage for the monitoring operations. Further, the monitoring method and apparatus would define at least one home position to facilitate operation of the printer.